Write docs for context-propagation

[chickenchickenlove]

From this PR, Aremeria will support micro-meter:context-propagation.
However, there is no documentation for context-propagation.

@minwoox recommend to write docs for context-propagation like comments below.

We could add a dedicated page about context propagation to the ADVANCED section on the Armeria website. The page would start by explaining context propagation using an Armeria RequestContext, followed by guidance on debugging potential leaks using the requestContextLeakDetectionSampler. It would then delve into Micrometer context propagation and the use of RequestContextHooks.